1. Field of the Invention
The present invention relates to a dielectric waveguide resonator and a dielectric waveguide filter for use particularly in a microwave or millimeter wave range, and to a method of adjusting the characteristics thereof.
2. Description of the Related Art
There are various types of dielectric resonators known for use in the microwave range. They include: a TE01.delta.-mode dielectric resonator consisting of a dielectric in the form of a solid circular cylinder or a hollow circular cylinder placed in a shield case; a TM110-mode dielectric resonator consisting of a prism-shaped dielectric which is placed in a metallic case or a case covered with a conducting film in such a manner that the dielectric extends from the upper to the lower faces of the case; and a TEM-mode dielectric resonator consisting of a dielectric wherein an inner conductor is disposed in the dielectric and the outer surface of the dielectric is covered with an outer conductor. These dielectric resonators have their own features and advantages and are used as microwave devices in various applications depending on particular purposes.
The size of these dielectric resonators can be reduced by confining the majority of resonating energy into a dielectric member and furthermore by forming a magnetic wall at a location close to a boundary plane between the dielectric member and air in such a manner that the magnetic wall is coincident with the even-mode symmetric plane. In these dielectric resonators, the resonance frequency and unloaded Q are determined by the size, shape, and dielectric constant of the dielectric resonator and the metallic case, and also by the location of the dielectric member in the metallic case.
In the case of a dielectric waveguide resonator consisting of a dielectric material such as a ceramic dielectric whose outer surface is covered with a conducting film, its size can be reduced by a factor of 1/.epsilon..sub.r +L relative to the size of a resonator in the form of a waveguide cavity where .epsilon..sub.r is the dielectric constant of the dielectric material. Thus, the dielectric waveguide resonator is expected to find applications in small-sized low-loss filters in the microwave and millimeter wave ranges. When a dielectric waveguide filter of such a type is combined with a microstrip line or a similar circuit element, the coupling between the dielectric waveguide filter and the external circuit is achieved by means of a structure such as those shown in FIGS. 33-35. In the example shown in FIG. 33, a conducting film 2 is formed on the outer surface of a dielectric block 1 so that the middle part of the dielectric block 1 serves as a waveguide system with a high Q, and coaxial TEM resonators are formed at either end of the dielectric block 1. In the example shown in FIG. 34, a conducting film 2 and stubs 9 are formed on the outer surface of a dielectric block wherein the coupling to the waveguide resonator system and the coupling to an external microstrip line are achieved via the stubs 9. In the example shown in FIG. 35, a hole is formed in a particular side of a dielectric block 1, and a probe 10 is inserted into the hole thereby achieving coupling to a waveguide resonance mode.
In the above-described conventional structures of dielectric resonators which operate in the TE01.delta., TM110, or TEM mode, the resonance frequency and unloaded Q can be rather easily set to desired values by properly selecting the external dimensions. However, these dielectric resonators have problems in design and production arising from their structure. That is, in the TE01.epsilon.-mode dielectric resonator, a complicated structure is required to dispose a dielectric resonator at a particular fixed location in a shield case. In the case of the TM110-mode dielectric resonator, it is not easy to connect a prism-shaped dielectric to a metallic case or a case covered with a conducting film through which a current flows. When the prism-shaped dielectric and the outer conductor are combined in an integral fashion, a complicated and difficult molding technique is required. Furthermore, it is required that an end of the case be open so as to process the prism-shaped dielectric in the case. When the resonator is used, it is required to cover the open end with a conductor. This causes an increase in the cost of the production and assembly process. On the other hand, in the case of a TEM-mode dielectric resonator, the outside dimensions should be great enough to obtain a high unloaded Q. However, if the outside dimensions are increased, the resonance frequency in a high-order resonance mode becomes close to the primary resonance frequency in the TEM mode to be used. Since only a certain number of dielectric materials are available in practical production, the unloaded Q is limited within a certain range. In the case where a band-pass filter is constructed of a dielectric block having a plurality of inner conductor holes and having a coupling hole formed in the middle of each inner conductor hole wherein the coupling between resonators is adjusted by properly selecting the effective dielectric constant between resonators, it is required that only the inner surface of each inner conductor hole be covered with an inner conductor while the inner surface of the coupling holes should remain uncovered. However, this requires a complicated production process.
It is also known in the art to construct a dielectric waveguide resonator by forming a conducting film on the outer surface of a ceramic dielectric. This structure is equivalent to a cavity resonator filled with a dielectric. If a dielectric with a dielectric constant of .epsilon..sub.r is employed, a reduction in wavelength occurs and thus it is possible to reduce the total size of the resonator by a factor equal to 1/.epsilon..sub.r +L . FIG. 31 illustrates the structure of a TE101-mode dielectric waveguide resonator. The wavelength inside the resonator is given by .lambda.g=2ac/a.sup.2 +L =c.sup.2 +L , and this wavelength determines the resonance frequency. The unloaded Q is determined by the wavelength .lambda.g, the skin depth .delta. of the conducting film formed on the surface of the dielectric, and the dimensions a, b, and c of the dielectric block wherein the unloaded Q increases with the dimensions a, b, and c. Although this type of dielectric waveguide resonator requires a greater size for the same resonance frequency than a coaxial dielectric resonator, it is easy to produce a resonator having a high unloaded Q. However, in this type of dielectric waveguide resonator, when the dielectric constant .epsilon..sub.r of the ceramic dielectric used and the main resonance frequency as well as adjacent resonance frequency are given, the dimensions a, b, and c of the resonator are determined by the given parameters, and the unloaded Q is determined by the dimensions a, b, and c. This requires the dielectric constant .epsilon..sub.r of the dielectric material to be within the range around 20, from 30 to 35, or around 90. In practice, it is difficult to freely select the dielectric constant. Therefore, when a desired resonance frequency is achieved using a given dielectric material, the only parameter allowed to vary to adjust the unloaded Q is the dimension b. In this case, it is required to properly select the dimension b while taking into account the effect of the adjacent resonance frequency on the main resonance frequency. Thus, this type of resonator is difficult to design and adjust.
In view of the above, it is an object of the present invention to provide a dielectric waveguide resonator whose resonance frequency and unloaded Q can be designed in a more flexible fashion, and can be easily adjusted to desired values.
FIG. 33 illustrates the structure of a conventional dielectric waveguide filter. Although this type of dielectric waveguide filter can be easily coupled to a microstrip line, the coaxial resonator portions have a low unloaded Q relative to that of the waveguide resonator, and thus the overall unloaded Q becomes low. On the other hand, in the case of the structure shown in FIG. 34, it is required that the length of the stub 9 should be large enough to achieve strong coupling. However, the long stub 9 can cause leakage of electromagnetic waves via the gap between the stub 9 and the conducting film 2. The leakage of electromagnetic wave can cause interference in an external circuit. In the structure shown in FIG. 35, it is required that a probe 10 should be prepared separately from the resonator. Furthermore, it is also required to securely fix the probe 10 relative to the dielectric block 1.
Thus, it is another object of the present invention to provide a dielectric waveguide resonator having a simple coupling circuit element by which coupling to an external circuit can be achieved without having to use an additional special member and without causing a great amount of leakage of electromagnetic waves toward the outside.